The present invention relates to continuous ink jet printing and, more particularly, to a startup sequence for a continuous ink jet printhead to transition from a lower pressure state to a final operating pressure state.
Ink jet printing systems are known in which a printhead defines one or more rows of orifices which receive an electrically conductive recording fluid from a pressurized fluid supply manifold and eject the fluid in rows of parallel streams. Printers using such printheads accomplish graphic reproduction by selectively charging and deflecting the drops in each of the streams and depositing at least some of the drops on a print receiving medium, while others of the drops strike a drop catcher device.
In normal operation of the printhead, the charging electrodes deflect most of the ink drops, causing them strike the catcher face. The ink then flows down the catcher face and enters the catcher throat. Vacuum then draws the ink through the catcher outlet port back to the ink reservoir. In designing the catcher ink return path it is important that the return path provide uniform ink removal along the entire length of the catcher throat. U.S. Pat. No. 6,187,212, totally incorporated herein by reference, describes an exemplary ink removal geometry. The flow path described therein provides uniform vacuum along the entire length of the catcher throat while keeping the pressure drops in the flow path to a minimum.
During the automatic startup sequence of a continuous ink jet printhead, the fluid pressure to the ink jets can be anywhere from a low pressure where ink xe2x80x9cweepsxe2x80x9d from the droplet generator to the final operating pressure. By way of example, for the Versamark printhead, the startup sequence includes states where ink weeps at low pressure from the droplet generator, to help redissolve ink on the exterior of the orifice plate and on the charging electrodes; states where ink is jetted out of the droplet generator orifices at 8 psi to allow condensate cleaning and drying of the charge plate; and states where the ink pressure is at the operating pressure of 15 psi, prior to turning on the drop charging to deflect the droplets onto the catcher face.
During the startup sequence, eyelid means are then used to seal against the bottom of the catcher. The eyelid sealing means not only seal against the catcher, but they are also designed to divert the ink that is jetting from the drop generator into the catcher throat. It has been determined that this process of diverting the ink flow into the catcher throat by means of the eyelid has much higher fluid flow energy losses than the process of having the ink drops strike the catcher face and then flow into the catcher throat. As a result, a catcher ink return geometry that can effectively remove ink from the printhead when the drops are deflected into catch may have too much restriction to remove ink that is diverted into the catcher throat by the eyelid. This can result in ink filling the space between the eyelid and catcher and eventually in ink overflowing out of this space. Enlarging the cross section of the ink return path can reduce the flow restrictions sufficiently to remove the ink diverted into the throat by the eyelid. During normal operation however, the lowered flow restriction in the ink return line can result in excessive air being drawn into the catcher throat. This can result in excessive amounts of foam being generated in the catcher return line and in the ink tank.
A catcher ink return geometry has been developed for some printheads which could provide acceptable ink removal both while the ink is charged and deflected into catch and during the startup sequence when the ink is diverted into the catcher throat by the eyelid. It has been determined, however, that sharp transitions in flow rate, such as are produced by stepping the ink pressure from 8 psi to 15 psi, could result in ink overflowing the space between the eyelid and the catcher. Therefore, it has been necessary to slowly ramp up of the ink pressure to avoid the problems caused by sharp flow transitions.
In newer printheads designed for high print speeds, the ink flow rates are much higher than prior art printheads, presenting difficulties not heretofore encountered in the art. For example, the 165 kHz printhead, developed and manufactured by Scitex Digital Printing, Inc., in Dayton, Ohio, operates at 28 psi and up to 1300 ml/min flow rate. The pressure is 87% higher and the flow rate is 73% more than in previous printheads. At such flow rates, it is not possible to adjust the catcher geometry to facilitate proper ink removal both when the ink drops are deflected into catch and when the ink must be diverted into the catcher throat by the eyelid. The catcher ink return fluid restrictions do not allow for adequate ink removal during startup states when the ink must be diverted into the catcher throat by the eyelid and the ink pressure is at the normal operating pressure. This results in ink overflowing the space between the eyelid and the catcher
It would be desirable, therefore, to be able to transition from a lower pressure state to a final operating pressure state without encountering the problems associated with the prior art.
The ink return problem finds its solution in the rapid pressure ramp of the ready startup cycle of the printhead, in accordance with the present invention. During the startup sequence when the eyelid must be used to divert the ink into the catcher throat, the ink pressure is kept below its normal level. This reduces the flow rate sufficiently to allow the ink to be adequately removed by the catcher ink return path. The ink pressure is then increased to the normal operating pressure and the charge voltage turned on to deflect the drops into catch in a time interval that is short enough to prevent the backup of ink in the printhead, between the eyelid and catcher.
In accordance with one aspect of the present invention, a method is provided for transitioning from a lower pressure state to a final operating pressure state. Initially, an eyelid is used to divert ink into a fluid channel associated with the catcher assembly. Pressure of the ink is reduced to a low ink pressure level that will allow the ink to be removed by the fluid channel. Pressure of the ink is increased to at least one incremental step, before reaching a final ink operating pressure. A charge voltage is turned on to deflect ink into catch in a time interval short enough to prevent ink backup between the eyelid and the catcher assembly.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.